This invention relates to a lens element consisting of a glass member on which a resin layer is integrally molded, and to a lens barrel in which such a lens element is incorporated.
In the prior art, a method is known in which a thin film comprising a resin material is molded on the surface of a glass member to form a lens element having an aspherical shape, which is difficult to work when the material is glass. The lens element molded by this method generally is referred to as a replica lens.
As shown in FIG. 1, the method of molding a replica lens includes placing a glass element 50, which has been worked into a spherical shape having a curvature approximating an aspherical shape, upon a mold member 52 having a molding surface 52a of a desired aspherical shape, filling a space 54 between a surface 50a of the glass member 50 and the molding surface 52a of the mold member 52 with a liquified resin and allowing the resin to harden, thereby forming a replica lens 55 having the desired aspherical shape.
The glass member 50 used in this molding method has a rim portion 50b for positioning the completed replica lens 55 relative to a lens barrel. In the prior art, the rim portion 50b is obtained by extending, in the circumferential direction, the surface 50a worked into a spherical shape in order to form the resin layer, or by working the circumferential portion of the glass member 50 into the shape of a flange.
In this example of the prior art, however, the resin material may overflow from the space 54 between the surface 50a of the glass member 50 and the molding surface 52a of the mold member 52, adhere to the rim portion 50b in the manner shown in FIG. 1, and solidify in this state. When the replica lens 55 completed in this fashion is mounted in a lens barrel, the replica lens 55 cannot be positioned accurately due to the protruding hardened resin material, and optical performance deteriorates as a result.
Furthermore, in order for the rim portion to be abutted against a positioning portion of the lens barrel to perform the function of positioning the replica lens with respect to the lens barrel, it is required that the rim portion be worked to a high precision. Consequently, in the case where the outer circumferential portion of the glass member 50 is worked to a flange-shaped configuration to form the rim portion, it is necessary that rim portion be worked to have a high precision. Such working of the glass member 50 requires labor and raises the cost of the replica lens.
In a case where the replica lens is incorporated in a lens barrel, the method adopted in the prior art involves fitting the outer circumferential surface of the replica lens into the inner peripheral surface of the lens barrel in a lightly press-fitted state, whereby the optic axis of the replica lens is aligned with the optic axis of the lens barrel, and subsequently securing the replica lens in the lens barrel by a counter spring. With this method of incorporating the replica lens, however, the inner peripheral surface of the lens barrel is shaved off by the outer circumferential surface of the replica lens when the replica lens is press fitted into the lens barrel. A problem which arises is that shavings produced attach themselves to the optical portions of the replica lens. Since the replica lens ordinarily is an insulator, the shavings are attracted by static electricity produced on the surface of the lens and therefore readily adhere to the lens.
In order to prevent unnecessary light rays reflected at the inner wall of the lens barrel for impinging upon the replica lens and detracting from the overall optical performance of the lens, the conventional practice is to provide the inner wall of the lens barrel with a light shield in such a manner that light reflected at the inner wall of the lens barrel will not impinge upon the lens element. Consequently, if the lens barrel is made of metal, working the light shield requires labor. In a case where the lens barrel is molded of a resin material, the shape of the mold is complicated. This is a hindrance to a reduction in the cost of the lens.
In the case where a resin layer is molded on the surface of the glass member 50 by the above-described method, the resin material contracts as it hardens. More specifically, before hardening occurs, the resin material is introduced so as to fill the space 54 between the molding surface 52a of the mold member and the surface of the glass member 50. After hardening, the resin contracts so that the volume thereof becomes smaller than the volume of the space 54 defined by the molding surface 52a and the surface of the glass member 50.
The following phenomena occur due to this reduction in the volume of the resin material:
(a) The glass member 50 itself flexes under a pulling force from the contracted resin. PA1 (b) The resin material peels off the surface of the glass member 50. PA1 (c) The resin material peels off the molding surface 52a of the mold member 52.
If the glass member 50 itself flexes as set forth in (a) above, the molded aspherical shape will merely undergo a slight shift from the shaped desired. However, if the resin material peels off the glass member 50 as set forth in (b) above, the result is a fatal defect. Further, in the case where the resin material peels off the molding surface 52a, as in (c) above, the entire resin material does not peel off uniformly but only partially. As a consequence, a difference in the ratio of contraction occurs between the portion of the resin material which has peeled off and the portion still clinging to the molding surface 52a. This causes the surface of the resin to be molded into a shape having a curvature which is discontinuous, with the borderline being the portion of the resin material which has peeled off and the portion still clinging to the molding surface.
With regard to the peeling phenomenon between the glass member 50 and the resin material in (b) and between the molding surface 52a and the resin material in (c), the ratio of peeling varies depending upon the type of resin material and the curing conditions. Accordingly, there are cases in which peeling can be prevented by changing these conditions. However, the ratio of peeling is largely dependent upon the shape to which the resin layer is to be molded, and there are instances where peeling cannot be prevented merely by changing the molding conditions.
A method proposed in the prior art as an expedient for preventing (b) and (c) involves enlarging the outer diameter of the glass member 50 to a size greater than necessary and supporting the glass member 50 at a position as close to its outer circumferential edge as possible, thereby intentionally enlarging the deformation [mentioned in (a) above] of the glass member 50 itself so as to prevent the peeling phenomenon.
However, when the outer diameter of the glass member 50 is enlarged in this manner, the lens barrel in which the glass member is mounted is increased in size and, hence, so is the camera or video camera which uses the lens.